Remove tabs UI from code editor and move to plugin. Enhance plugin system.

- Remove tabs controller, tab widget, and tabs widget files and move to plugin
- Delete plugins/README.txt
- Add register_controller method to controller system for plugin use
- Add error handling for plugin crashes via futures callback

src/plugins/plugin_context.py

@@ -38,3 +38,7 @@ class PluginContext:
 
     def emit_and_await(self, event_type: str, data: tuple = ()):
         raise PluginContextException("Plugin Context 'emit_and_await' must be overridden...")
+
+    def register_controller(self, name: str, controller):
+        raise PluginContextException("Plugin Context 'register_controller' must be overridden...")
+